1. Field of the Invention
This invention relates to electrochemical cells and more particularly relates to a gel containing an electrolyte.
2. Background of the Invention
Devices that convert chemical energy into electrical energy are commonly referred to as batteries. Primary batteries contain compounds of high chemical energy which are converted to compounds of low chemical energy when electrical energy is withdrawn. These batteries are discarded when exhausted. Fuel cells are a special class of primary batteries.
With secondary batteries, electrical energy may be put back into the battery to convert the low energy compounds back to the high energy compounds. This process, known as recharging, allows secondary batteries to be used over and over.
Batteries may be of one or more individual electrochemical cells. A cell includes a negative electrode (anode) and a positive electrode (cathode) separated by an electrolyte solution. Ions migrate through the electrolyte solution between the anode and the cathode.
Commonly used liquid electrolytes are sulfuric acid in lead-acid batteries and potassium hydroxide in nickel-cadmium and silver-zinc batteries. Liquid electrolytes have the advantages of excellent contact between the electrodes and the electrolyte solution and ease of transport of ionic species through the solution so that very high conductivity is achieved. (In the present disclosure, conductivity is given by the term S.times.cm.sup.-1 wherein S designates siemans (reciprocal ohms)). Disadvantages associated with liquid electrolytes are the potential for leakage and the requirement that the battery be used in a prescribed (usually vertical) position.
Electrically conductive solids with ionic carriers have been disclosed for potential applications where liquid electrolytes cannot be used. Most solid electrolytes are inorganic materials such as ceramics or compacted tablets. With such materials, difficulties are often encountered in maintaining close contact with the electrodes and in developing sufficient conductivity.
One type of solid electrolyte which has been proposed is a polymer containing ions. Exemplary of these polymeric electrolytes are ionic species such as hydrated perchlorates in polyacrylonitrile, polyethyleneoxide, and polyvinylidene fluoride. Conductivities in the range of 10.sup.-7 to 10.sup.-2 S cm.sup.-1 have been reported, and deformation under stress which helps maintain close electrolyte-electrode contact has been claimed.
Gels have also come into use to immobilize electrolyte systems in batteries for consumer applications, such as emergency lighting and alarm-security systems. Exemplary of such systems is a thixotropic gel obtained by mixing sodium silicate, demineralized water and dilute sulfuric acid.
While much progress has been made in overcoming the disadvantages of liquid electrolytes by using solid electrolytes and crosslinked gels, there remains a need for a self-supporting electrolyte having the conductivity of a liquid electrolyte. It is toward solution of this problem that this invention is directed.